Many electronic devices, such as telecom devices, incorporate semiconductors in their design and operation. Semiconductors can be differentiated from metals and insulators. The behavior of valence, or unbonded electrons in a given material helps to determine whether that material acts as a metal, an insulator, or a semiconductor. Electrons in a material occupy different quantum, or energy states, depending on factors such as temperature and the absence or presence of an externally applied electrical potential. The highest energy quantum state occupied by an electron for a given material, while that material is at 0°K is known as the Fermi energy, EF.
In metals, the Fermi energy, EF falls in the middle of a band of allowed quantum states, closely spaced in energy. As a result, this means that an infinitesimally small voltage allows an electron to be promoted from lower energy quantum states to higher energy quantum states. Therefore, electrons may move freely through metals. The ability to easily permit the movement of electrons in a material allows metals to carry an electrical current. As such, metals are excellent conductors.
For insulators, the Fermi energy, EF falls inbetween widely spaced quantum energy states. As a result, when compared to metals, a comparatively large voltage is required to promote an electron to a more energetic level. Electrons in insulators are much less mobile and can carry far less current than metals in response to a given voltage.
Semiconductors are similar to insulators in that the Fermi energy, EF falls inbetween spaced quantum energy states. However, the gap between these energy states in a semiconductor is more narrow than the gap for an insulator. This allows electrons in semiconductors to be promoted by external energy from quantum states in the lower-energy valence bands to quantum states in the higher-energy valence bands. The ability of electrons in semiconductors to be promoted from one quantum state to another provides the electron mobility needed for current flow.
Promotion of an electron produces a negatively charged mobile conduction band electron, or free electron, as well as a positively charged hole in the valence band. Both the free electron and the hole are mobile charge carriers that support the flow of current. The density of positive or negative charge carriers in a semiconductor can be increased by adding ionized impurities, or dopants, to a semiconductor. A semiconductor material with no added impurities is referred-to as an intrinsic semiconductor. A semiconductor material with added dopants is referred-to as an extrinsic semiconductor. An extrinsic semiconductor with an increased density of positive charge carriers, or holes, is referred-to as a p-type semiconductor. An extrinsic semiconductor with an increased density of negative charge carriers, or free electrons, is referred-to as an n-type semiconductor.
Transistors and other semiconductor devices are based on junctions between different semiconductor materials of different properties. In heterojunctions, regions of different bulk semiconductor materials are joined at an interface. For example, n-type semiconductors may be interfaced with p-type semiconductors. In homojunctions, regions of the same bulk semiconductor (all n-type, or all p-type), each with possibly different levels or types of dopants to produce different semiconductor parameters, are joined at an interface.
At the interface, or junction between two semiconductor materials, a depletion region forms due to the movement of free electrons from the n-type region into the adjoining p-type region, where the free electrons combine with the holes. This effectively collapses the free electrons and electron holes into bound valence electrons. These bound valence electrons in the depletion region result in a potential energy barrier against the migration of additional free electrons from the n-type material into the p-type material.
A forward bias may be applied to the semiconductor materials by connecting a positive end of a voltage to the p-type material and the negative end of the voltage to the n-type material. As the forward bias is increased, the depletion region narrows and eventually does not exist. At this point, as the voltage is further increased, current will begin to flow between the semiconductor materials. When the forward bias is removed, or reduced to the point where the depletion region exists again, current will not flow between the semiconductor materials.
Semiconductors are often incorporated in the construction of microcircuit devices. A given microcircuit may have bipolar transistors, metal-oxide semiconductor (MOS) transistors, diodes, resistors, or any combination thereof Bipolar transistors have at least three semiconductor regions: A base of a first type of semiconductor material, and a collector and an emitter of a second type of semiconductor material. Microcircuits which incorporate bipolar transistors are often fabricated using silicon (Si) based materials and processes. Maximizing Si-based bipolar transistor performance is a goal of the Si integrated circuit industry. In furtherance of this goal, the vertical dimensions of bipolar transistors are being scaled back. The scaling-back may result in certain device operational limits. For example, when the base thickness is decreased, the doping level must be increased in order to control the depletion region and maintain a low base resistance. Unfortunately, increasing the doping level of the base decreases the gain (and therefore the utility) of the bipolar transistor.